In an image display device, an image consists of a large number of image points, each having a specific grey value or color and a specific brightness. In a specific class of image display devices, a viewer is watching a display screen behind which a light source is arranged, the so-called backlight. The display screen comprises a plurality of pixels which can be controlled to pass light or to block light. In a specific embodiment, a pixel is implemented as a liquid crystal cell. A controller receives a video signal with image data, and on the basis of these image data it generates control signals for the liquid crystal cells. In the following, a control signal for pixel cells will be indicated as SCP, and it will be assumed to have a minimum value 0 and a maximum value 1.
The image data can range from perfect black to perfect white. The image data are translated by the controller to a certain value for the control signal SCP. In the case of perfect black, the brightness data in de video signal will be assumed to have a minimum value 0. It is noted that, in response to receiving a control signal SCP=0, the pixel cell should block all light from the backlight. In practice, however, a pixel cell will always “leak” to some extent. In the case of perfect white, the brightness data in de video signal will be assumed to have a maximum value 1. It is noted that, in response to receiving a control signal SCP=1, the pixel cell should pass all light from the backlight. In practice, however, a pixel cell will always reflect and/or absorb to some extent. So, generally speaking, the transmission rate of a pixel cell, indicated as H, will range from a minimum value α to a maximum value β, wherein 0<α<β<1.
In an actual image, the darkest portions may be lighter-than-black and the brightest portions may be darker-than-white. Thus, the transmission rate for all pixels of the image will be in a range from α* to β*, with α<α*<β*<β. The values α* en β* determine the contrast of the image: a high contrast ratio means that the distance between α* and β* is as large as possible.
Apart from the actual value of the transfer rate H, the amount of light IP emanating from a pixel, as viewed by the viewer, depends on the brightness of the backlight, in other words the intensity IBL of the light generated by the backlight. This might be expressed in a formula as follows:IP=H·IBL  (1)
Thus, with a certain setting of the intensity IBL of the backlight, the brightness IP of a pixel can range from α·IBL to β·IBL.
Under certain circumstances, it may be desirable to increase the light output. This may for instance be the case if the level of the ambient light is relatively high. Increasing the light output may be done by shifting the range [α*,β*] to higher values, or at least by shifting the upper limit β* of this range to higher values.
On the other hand, under certain other certain circumstances, it may be desirable to decrease the light output. This may for instance be the case if the level of the ambient light is relatively low. Decreasing the light output may be done by shifting the range [α*,β*] to lower values, or at least by shifting the lower limit a* of this range to lower values.
However, increasing or decreasing the light output can also be achieved by increasing or decreasing the intensity IBL of the backlight.
From formula 1, it follows that the same pixel brightness IP can be achieved for different settings of the brightness IBL of the backlight. If the brightness IBL of the backlight is multiplied by a certain factor X, and simultaneously the transfer rate H of a pixel cell is divided by the same factor X, the resulting product (X·IBL)·(H/X)=IP. This fact is utilized in backlight boosting and backlight dimming.
In the case of backlight boosting, the intensity IBL of the backlight is increased. This can be used to enhance white parts of an image. By increasing the backlight intensity IBL, those parts appear to be “better white” for the viewer. In grey parts of the image, the grey value can be maintained by simultaneously reducing the control signal SCP for the pixel cells, so that the pixels cells pass less light.
In the case of backlight dimming, the brightness IBL of the backlight is decreased. This can be used to enhance black parts of an image. By decreasing the backlight intensity IBL, those parts appear to be “better black” for the viewer. In grey parts of the image, the grey value can be maintained by simultaneously increasing the control signal SCP for the pixel cells, so that the pixels cells pass more light.
By alternating backlight boosting and backlight dimming, the overall contrast ratio of the display device can be enhanced, and energy can be saved.
An image display device is designed for a certain nominal setting of the backlight light source. In this nominal setting, the backlight light source consumes a certain amount of power, and consequently it generates a certain amount of heat; the image display device is designed to handle this amount of heat. It should be clear that changing the contrast range [α*,β*] of the transmission rate of the screen pixels does not change the power consumption of the backlight. When using backlight dimming, energy is saved, but when using backlight boosting, the backlight light source produces more heat than the image display device is designed to handle. If this situation continues for a prolonged amount of time, the apparatus may become too hot. This problem might be mitigated by using additional cooling means, but this would add to the hardware costs and the energy bill of the apparatus.